Clinical oncology and cancer biology are challenged by the lack of assay platforms for measuring changes in cancer-cells'growth kinetics in response to therapeutic intervention. Cell growth kinetics can be measured in a number of ways, such as by DNA copy number, volume, mass, density, shape, or by expression of particular proteins. Here we define growth kinetics as changes in a cell's mass and density over time. We propose to develop an instrument for concurrently monitoring single cell growth kinetics and cell surface protein expression. We hypothesize that changes in growth kinetics and in cell surface protein expression can be used as a surrogate for response to pathway-directed therapeutic agents. As a validation of the instrument, we will monitor mass, density and cell-surface protein expression (determined by fluorescence) in single A431 cells in response to intervention with the pathway targeted therapy gefitinib, a small molecule inhibitor of the epidermal growth factor receptor (EGFR), and appropriate controls. Single cell mass and density will be measured by a previously validated device known as the suspended microchannel resonator (SMR). The SMR can measure the mass of a mammalian cell with a resolution near 0.01% (1 Hz bandwidth). In order to achieve a continuous measurement of mass and density, silicon posts will be used for capturing a single cell within the sensitive region of the microchannel resonator. To enable simultaneous detection of protein expression, by fluorescence of coupled antibodies, the SMR will be modified to have optical transparency within the region where the cell is captured. The proposed instrument will leverage our prior proteomics work with the Center for Cancer Nanotechnology Excellence focused on Therapeutic Response (CCNE-TR) where we used proteomic methods to discover cell-surface protein biomarkers indicative of therapeutic response. Single-cell measurement of the abundance of these proteins, concurrently with cell mass and density kinetics will provide a new approach for characterizing and monitoring cell response to therapy on a physiological and molecular level.